As is well known, air which has been in contact with solvents or other volatile liquids will contain a variable quantity of solvent or liquid vapour, to the point of saturation, dependent upon the temperature of that air.
The common method used to remove the solvent vapour contained in the air-stream from a dry cleaning machine is to pass the solvent laden air-stream through a bed of activated carbon. The carbon adsorbs the solvent vapour held in the air-stream, allowing the cleaned air to pass through the carbon bed to atmosphere. The carbon bed will only adsorb approximately one gallon of solvent per 80 lb of carbon before becoming saturated with solvent vapour. The carbon bed must then be de-adsorbed by passing a "blanket" of steam through the carbon bed in a reverse direction to that of adsorption. The steam and solvent vapours form an azeotrope which must then be condensed, and the resultant water and liquid solvent separated according to their specific gravities. The stream of gas may then be recycled.
Another known method particularly suitable for use when the solvent is perchloroethylene, is to pass the solvent laden stream over a cooling coil so that the solvent condenses and can be collected for recycling. The stream of gas may also be recycled. This is possible when the solvent is trichloroethane or trichloroethylene but insufficient condensation can be achieved under normal operating conditions and it has been common practice to discharge the solvent laden stream to atmosphere.